Optical memory technology employing an optical disc with a pit pattern as a high-density and large-capacity storage medium has been put to practical use while expanding its uses to digital audio discs, video discs, document file discs, and data file discs. Functions which are necessary for successfully carrying out recording and reading of data in/from an optical disc with a high reliability, by using finely-condensed laser light, are generally classified into: a converging function of forming a diffraction-limited light spot; a focal point control (focus servo) function and a tracking control (tracking servo) function of an optical system; and a pit signal (data signal) detection function.
In recent years, advancement of optical system designing techniques and shortened wavelengths of semiconductor laser devices as light sources have enhanced development of optical discs which have higher recording densities and larger storage capacities than conventional optical discs. One of the approaches which are currently studied for higher densities is increasing the numerical aperture (NA) of a light-condensing optical system which is employed for forming a very small spot of condensed laser light on an optical disc. A disadvantage of this approach is increase in the amount of aberration caused due to the inclination (so-called “tilt”) of the optical axis. As the NA is increased, the amount of aberration caused due to the tilt increases. This can be avoided by decreasing the base layer thickness of a substrate of an optical disc, i.e., the thickness of a transparent cover layer.
Compact Discs (CD), which are so-called first-generation optical discs, are used with infrared light (wavelength λ3: 780 nm to 820 nm) and an objective lens of NA=0.45, and have the disc base layer thickness of 1.2 mm. DVDs (Digital Versatile Discs) of the second generation are used with red light (wavelength λ2: 630 nm to 680 nm, typical wavelength: 650 nm) and an objective lens of NA=0.6, and have the disc base layer thickness of 0.6 mm. The third-generation optical discs (Blu-ray Discs) are used with blue light (wavelength λ1: 390 nm to 415 nm, typical wavelength: 405 nm) and an objective lens of NA=0.85, and have the disc base layer thickness of 0.1 mm.
In this specification, the base layer thickness (substrate thickness) refers to a thickness from a laser light entry surface to a data layer of an optical disc which is a data medium.
As described above, the base layer thickness of the high-density optical discs is small. In view of economies and the space occupied by an apparatus, an optical data apparatus is desirable which is capable of recording and reading data in/from a plurality of types of optical discs having different base layer thicknesses and different recording densities. Such an apparatus cannot be realized without an optical head device which includes a light-condensing optical system capable of condensing laser light to the diffraction limit on optical discs of different base layer thicknesses.
In the case where data is recorded in or read from a disc which has a greater base layer thickness, it is necessary to condense laser light on a data layer which is at a deeper level than the disc surface. Therefore, it is necessary to increase the focal length.
A configuration designed for the purpose of realizing multiple compatibility reading or multiple compatibility recording in/from an optical disc having a base layer thickness of 0.6 mm with applicable wavelength λ2 (red light) and an optical disc having a base layer thickness of 0.1 mm with applicable wavelength λ1 (blue light) has been proposed. Patent Document 1 and Non-patent Document 1 disclose configurations in which a wavelength selecting phase plate is combined with an objective lens. This configuration is described with reference to FIG. 37 and FIG. 38.
FIG. 37 shows a configuration of an optical head device. Collimated light emitted from a blue light optical system 51 which includes a blue light source of emission wavelength λ1=405 nm is transmitted through a beam splitter 161 and a wavelength selecting phase plate 205, and condensed by an objective lens 50 on a data layer of an optical disc 9 (third-generation optical disc) which has a base layer thickness of 0.1 mm. Light reflected by the optical disc 9 returns the path it has come and is detected by a detector of the blue light optical system 51. Divergent light emitted from emitted from a red light optical system 52 which includes a red light source of emission wavelength λ2=650 nm is reflected by the beam splitter 161, transmitted through the wavelength selecting phase plate 205, and condensed by the objective lens 50 on a data layer of an optical disc 10 (second-generation optical disc: DVD) which has a base layer thickness of 0.6 mm. Light reflected by the optical disc 10 returns the path it has come and is detected by a detector of the red light optical system 52.
The objective lens 50 is designed such that collimated light incident thereon is condensed via the base layer thickness of 0.1 mm. In the case of recording or reading of data in/from a DVD, a spherical aberration is caused by a difference in base layer thickness. To correct this spherical aberration, the laser light emitted from the red light optical system 52 is divergent light, and the wavelength selecting phase plate 205 is used. When the divergent light enters the objective lens 50, another spherical aberration occurs. The spherical aberration caused by the difference in base layer thickness is canceled by this another spherical aberration, while the wavefront is corrected by the wavelength selecting phase plate 205.
FIG. 38(a) is a plan view of the wavelength selecting phase plate 205. FIG. 38(b) is a side view of the wavelength selecting phase plate 205. The wavelength selecting phase plate 205 is formed by a phase step structure 205a which has heights h and 3h where h=λ1/(n1−1) and n1 is the refractive index for wavelength λ1. The optical path difference for light at wavelength λ1 which is caused by height h is equal to wavelength λ1, which corresponds to phase difference of 2π. Therefore, it is equivalent to the phase difference of 0. Thus, this element does not affect the phase distribution and does not affect recording or reading of data in/from the optical disc 9. On the other hand, as for the light at wavelength λ2, the wavelength selecting phase plate 205 generates the optical path difference of h/λ2×(n2−1)≈0.6λ where n2 is the refractive index of the wavelength phase plate 205 for wavelength λ2, i.e., the optical path difference which is not an integral multiple of wavelength λ2. The phase difference caused by this optical path difference is utilized for the above-described correction of the aberration.
Patent Document 2 discloses a configuration in which a refractive objective lens and a diffraction element are combined. In this example, in an optical head device which performs recording or reading of data in/from a high-density optical disc using an objective lens of a large NA, a saw-tooth shape diffraction element is used for recording or reading of data in/from a conventional type optical disc, such as a DVD. The height of the saw-tooth for blue light is equal to optical path length 2λ, and the 2nd order diffracted light is used. In the red light, the 1st order diffraction occurs. Blazing direction is a convex lens shape, and the chromatic aberration of a refractive lens is corrected. In this case, the red light has a lower diffraction order, and therefore, a concave lens action is relatively exerted, so that the working distance can advantageously be increased.
Patent Document 2 also discloses a configuration wherein the cross-sectional shape of the grating is a stepped shape, one step difference of the stepped cross-sectional shape is an integral multiple of the unit step difference, and the unit step difference provides the first laser light at wavelength λ1 with the optical path difference which is equal to about 1.25 times the wavelength. This configuration is shown in FIG. 39(a).
Wavelength λ1 is from 390 nm to 415 nm. The shape of the grating for one period is a stepped shape wherein the steps sequentially have heights equals to multiples of step difference d1 by factors of 0, 1, 2, and 3 from the outer perimeter side to the optical axis side of the diffraction element. The grating changes the phase of the blue light in the same direction as that of the grating shape as shown in FIG. 39(b) so as to exert a convex lens action on the blue light. The grating changes the phase of the red light in the opposite direction to that of the grating shape as shown in FIG. 39(c) so as to exert a concave lens action on the red light. Therefore, to the blue light, the grating provides the effect of correcting the chromatic aberration caused by the refractive lens. At the same time, as for the red light, the effect of increasing the working distance (the space between the objective lens surface and the optical disc surface) by means of a concave lens action is achieved.
Patent Document 3 proposes that a relay lens is interposed between an infrared light source and an objective lens to achieve multiple compatibility using infrared light (wavelength λ3: 780 nm to 820 nm) and the objective lens of NA 0.45, with first-generation optical discs having a base layer thickness of 1.2 mm and with optical discs of different types.
Next, disc tilt solutions are described. Patent Document 4 proposes tilting an objective lens as a solution to a disc tilt. This is described with reference to FIG. 40. The outputs of a photodetection region 263 and a photodetection region 264 of a two-division photodetector 262 are calculated by a differential amplifier 265. The resultant signal is converted to a digital signal by an A/D converter 266. The digital signal is processed by an arithmetic processing unit 267. The resultant signal is converted by a D/A converter 268 to an analog signal. Based on the analog signal, a driver circuit 269 drives an actuator 270 to tilt an unshown objective lens. An example of the actuator and the objective lens is disclosed in Patent Document 5 and is herein described with reference to FIG. 41. Currents If1 and If2 which depend on focus error signals flow through focusing coils 201 and 202, respectively. The magnetic fields generated by these currents are received by a magnet 204, so that a lens holder 203 on which an objective lens 206 is mounted moves up and down. By controlling the magnitudes of currents If1 and If2, the lens holder 203 and the objective lens 206 are tilted right and left in the drawing.
Next, a decreased thickness of the base layer of high-density optical discs, which is different from the base layer thicknesses of low-density optical discs, and different optimum numerical apertures are considered. In view of economies and the space occupied by an apparatus, an optical data apparatus is desirable which is capable of recording and reading data in/from a plurality of types of optical discs which have different base layer thicknesses and different recording densities. Such an apparatus cannot be realized without an optical head device which includes a light-condensing optical system capable of condensing laser light to the diffraction limit using different numerical apertures on optical discs of different base layer thicknesses.    [Patent Document 1] Japanese Laid-Open Patent Publication No. H10-334504    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2004-071134    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2004-281034    [Patent Document 4] Japanese Laid-Open Patent Publication No. H10-312565    [Patent Document 5] Japanese Laid-Open Patent Publication No. H5-114154    [Non-patent Document 1] ISOM2001 Session We-C-05 (Precedings, page 30)